1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML
><HEAD
><TITLE
>Creating a swap space</TITLE
><META
NAME="GENERATOR"
CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK
REL="HOME"
TITLE="Linux System Administrators Guide"
HREF="index.html"><LINK
REL="UP"
TITLE="Memory Management"
HREF="memory-management.html"><LINK
REL="PREVIOUS"
TITLE="What is virtual memory?"
HREF="vm-intro.html"><LINK
REL="NEXT"
TITLE="Using a swap space"
HREF="using-swap.html"></HEAD
><BODY
CLASS="SECT1"
BGCOLOR="#FFFFFF"
TEXT="#000000"
LINK="#0000FF"
VLINK="#840084"
ALINK="#0000FF"
><DIV
CLASS="NAVHEADER"
><TABLE
SUMMARY="Header navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TH
COLSPAN="3"
ALIGN="center"
>Linux System Administrators Guide: </TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="bottom"
><A
HREF="vm-intro.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="80%"
ALIGN="center"
VALIGN="bottom"
>Chapter 6. Memory Management</TD
><TD
WIDTH="10%"
ALIGN="right"
VALIGN="bottom"
><A
HREF="using-swap.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="SWAP-SPACE"
></A
>6.2. Creating a swap space</H1
><P
>A swap file is an ordinary file; it is in no way special
to the kernel. The only thing that matters to the kernel is that it
has no holes, and that it is prepared for use with
<B
CLASS="COMMAND"
>mkswap</B
>. It must reside on a local disk, however;
it can't reside in a filesystem that has been mounted
over NFS due to implementation reasons.</P
><P
>The bit about holes is important. The swap file reserves
the disk space so that the kernel can quickly swap out a page
without having to go through all the things that are necessary
when allocating a disk sector to a file. The kernel merely
uses any sectors that have already been allocated to the file.
Because a hole in a file means that there are no disk sectors
allocated (for that place in the file), it is not good for the
kernel to try to use them.</P
><P
>One good way to create the swap file without holes is through
the following command:
<TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>$</TT
> <TT
CLASS="USERINPUT"
><B
>dd if=/dev/zero of=/extra-swap bs=1024
count=1024</B
></TT
>
<TT
CLASS="COMPUTEROUTPUT"
>1024+0 records in
1024+0 records out</TT
>
<TT
CLASS="PROMPT"
>$</TT
>
</PRE
></FONT
></TD
></TR
></TABLE
>
where <TT
CLASS="FILENAME"
>/extra-swap</TT
> is the name of the swap
file and the size of is given after the <TT
CLASS="LITERAL"
>count=</TT
>.
It is best for the size to be a multiple of 4, because the
kernel writes out <I
CLASS="GLOSSTERM"
>memory pages</I
>, which
are 4 kilobytes in size. If the size is not a multiple of 4,
the last couple of kilobytes may be unused.</P
><P
>A swap partition is also not special in any way. You create
it just like any other partition; the only difference is that
it is used as a raw partition, that is, it will not contain any
filesystem at all. It is a good idea to mark swap partitions
as type 82 (Linux swap); this will the make partition listings
clearer, even though it is not strictly necessary to the
kernel.</P
><P
>After you have created a swap file or a swap partition, you
need to write a signature to its beginning; this contains some
administrative information and is used by the kernel. The
command to do this is <B
CLASS="COMMAND"
>mkswap</B
>, used like this:
<TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>$</TT
> <TT
CLASS="USERINPUT"
><B
>mkswap /extra-swap 1024</B
></TT
>
<TT
CLASS="COMPUTEROUTPUT"
>Setting up swapspace, size = 1044480
bytes</TT
>
<TT
CLASS="PROMPT"
>$</TT
>
</PRE
></FONT
></TD
></TR
></TABLE
>
Note that the swap space is still not in use yet: it exists,
but the kernel does not use it to provide virtual memory.</P
><P
>You should be very careful when using
<B
CLASS="COMMAND"
>mkswap</B
>, since it does not check that the
file or partition isn't used for anything else. <EM
>You
can easily overwrite important files and partitions with
<B
CLASS="COMMAND"
>mkswap</B
>!</EM
> Fortunately, you should
only need to use <B
CLASS="COMMAND"
>mkswap</B
> when you install
your system.</P
><P
>The Linux memory manager limits the size of each swap space to
2 GB. You can, however, use up to
8 swap spaces simultaneously, for a total of 16GB.
</P
></DIV
><DIV
CLASS="NAVFOOTER"
><HR
ALIGN="LEFT"
WIDTH="100%"><TABLE
SUMMARY="Footer navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
><A
HREF="vm-intro.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="index.html"
ACCESSKEY="H"
>Home</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
><A
HREF="using-swap.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>What is virtual memory?</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="memory-management.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>Using a swap space</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>
|
